1. Field of the Invention
The present invention relates to a light stabilized polyamide substrate and to a process for making the same. More particularly, this invention relates to a film, fiber, or article made from a polyamide polymer having a specific hindered amine light stabilizer chemically bound thereto during polymerization to thereby enhance light stability, especially when the articles have been dyed with photoreactive chemicals or dyes.
2. Prior Art
It is known to use dyed fabrics woven from organic polymer fibers such as polyamides in flags, decorative banners and as covers for personal flotation devices, such as life preservers. Prolonged exposure to sunlight (weathering) has a tendency to fade the dyed polyamide material and to reduce its physical properties such as strength retention and resistance to elongation. This occurs when the dye and polymer components of the material absorb damaging wavelengths of light.
It is also known to use clear, transparent polyamide films for packaging food and other items. Prolonged exposure of these films to light also reduces physical properties, which is manifested by reduced flexibility, increased opaqueness and crazing, and crumbling.
Various organic light stabilizers are known for use with polymer substrates such as films, fibers and articles manufactured therefrom. One of the more effective organic light stabilizers has been the hindered amine light stabilizer. The two most common methods of incorporating the light stabilizers are by mixing with the pre-formed polymer and by coating, dipping or overspraying the film, fiber or woven Fabric. Many of the commercially available light stabilizers may not have sufficient thermal stability to survive melt incorporation into the polymer or downstream processing, especially with a polyamide such as nylon-6 where processing temperatures may exceed 270.degree. C. Other typical difficulties arising during the melt processing of additives such as UV light stabilizers in polymers include the degradation of polymer molecular weight, volatilization, leaching, extruder screw lubrication and the formation of undesired color. Coating, dipping or overspraying typically requires the stabilizer be applied in solution, dispersion or emulsion form, after the film, fiber or article has been produced. This type of application has the potential advantage of concentrating the stabilizer where it is most effective. However, the solvents and/or other liquids used as carriers for the stabilizer can pose environmental and workplace hazards.
U.S. Pat. No. 4,356,307 discloses the use of cyclic imides which are derivatives of 4-amino-2,2,6,6-tetramethylpiperidine for stabilizing polymers, especially polyolefins, against photo and/or thermal degradation. The stabilizers are incorporated after polymerization by mixing with the pre-formed polymer by techniques conventional in the art, and are thus susceptible to migration, leaching, and volatilization with a concomitant deterioration in stability of the polymer.
Both U.S. Pat. No. 4,354,007 and U.S. Pat. No. 4,743,657 teach methods for preparing polymer bound stabilizer precursor molecules which comprises reacting the molecules with a pre-formed polymer, which can be polyamide, in the presence of a free radical. In the latter reference, the stabilizer precursor molecule contains a reactive double bond which is not readily homopolymerizable, and the stabilizer precursor molecule can be a hindered piperidine. In both references the stabilizer molecules are reacted with a pre-formed polymer and thus require an additional process step.
The concept and utility of using bound stabilizers to control stabilizer location in multi-phase polymer systems, specifically attachment of a hindered piperidine to the rubber and matrix phases of AES terpolymer, has been demonstrated. J. Hahnfeld et al., Polymer-bound hindered amine light stabilizers for improved weatherability in multi-phase polymer systems, 39 Polymer Degradation and Stability 241-49 (1993). And PCT WO 92-12201 discloses the addition of UV light stabilizers, e.g., hindered amine light stabilizers, to polymeric compositions prior to the polymerization reaction.
It is therefore advantageous to have an efficient, environmentally friendly process for preparing light stabilized polyamide substrates such as molded objects, films, fibers and fabrics, and in particular flags, decorative banners, and fabric covers for personal flotation devices, wherein the light stabilizer is chemically bound to the polyamide and thus no longer susceptible to migration, leaching, and/or volatilization. It is also advantageous to have a process that utilizes an existing manufacturing process to accomplish one or more of its steps, and particularly advantageous to have a process wherein the light stabilizer readily substitutes functionally for one or more molecular weight regulators currently used during polymerization. The present invention achieves all of this.